Optical multipass cells are often considered in terms of a typical application employing such cells, such as for in use in optical absorption or emission spectrometers. Absorption spectroscopy, for example, is widely used in many applications, including environmental monitoring, greenhouse gas and pollution monitoring, and industrial process control applications in the energy and manufacturing sectors. In gas-phase absorption spectroscopy, for example, a gas absorption spectrometer detects the presence of a targeted-gas within a subject gas-sample by passing a light beam at a selected wavelength(s) through the gas sample, and measuring the absorption (attenuation) of the light beam as a function of the light wavelength, thus enabling the quantitative determination of the concentration of the targeted-gas or gases. As the level of attenuation of the light is directly proportional to the optical path length through the gas, absorption spectroscopy sensitivity, for example, can be enhanced by increasing the effective path length via the use of an optical multipass cell. An equation describing this absorption rate as a function of path-length (L), know as Beer's Law, is given byI(L)/Io=e−σcL,where Io is the initial intensity of the light, a is the absorption cross-section at the selected frequency for the absorbing species, and c is the concentration of the absorbing species.
Absorption spectroscopy is, however, merely one example of various applications of a further included optical multipass system referred to as an optical multipass cell. A typical optical multipass cell (also referred to herein as a “multipass cell”) broadly includes a particular arrangement of statically and/or adjustably mounted optics that provide for receiving and directing a provided light beam (also referred to herein as a “light source”) through a multipass cell inlet and cell cavity, and then outputting the result back through the inlet (the inlet also serving as an outlet) or through a separate outlet, and/or toward a window and/or so-called optical detector. In one common arrangement, the multipass cell comprises an elongated cylinder in which “end” mirrors are disposed at opposite ends of the cylinder; in some examples, only one of the end-mirrors defines a hole and the hole serves as both an inlet and outlet for the light beam, while in other examples, one end-mirror defines an inlet hole (or “inlet”) for the light beam and the other end-mirror defines an outlet hole (or “outlet”) for the light beam. In either case, a source light beam entering through the cell inlet is reflected along an optical path that directly connects the first end-mirror and the other end-mirror in a repetitive fashion before exiting through the outlet hole. The source light beam thus bounces repeatedly from one end-mirror directly to the other end-mirror until exiting through the outlet. The bouncing of the light beam directly between the end-mirrors acts to increase the optical length and exposure of the light to the gas or gases within the multipass cell without instead increasing the length of the cell. Multipass cells also typically employ at least one end-mirror that is concave in order to continually re-focus the light beam as it traverses the cell, thereby allowing for a stable optical pattern to be produced, and avoiding overlap and fringing/interference effects that might otherwise occur as the beam expands due to intrinsic divergence or “spreading” of the light beam.
Two underlying optical multipass cell types have long remained prevalent: the Herriott cell and the White cell. A Herriott cell type provides two opposing end-mirrors that are arranged at opposite ends of the multipass cell cavity and directed at one another. Thus, a source light beam is caused to repeatedly bounce across the length of the cell cavity—directly from one end-mirror to the other end-mirror—and thereby increasing the effective path length each time the light beam undergoes another reflection at each end mirror.
A White cell differs from a Herriott cell in that the White cell instead typically utilizes a three mirror arrangement; here two smaller concave mirrors are arranged at a first end of the multipass cell cavity so as to oppose a larger concave mirror at the second end of the multipass cell cavity. In this case, a source light beam bounces (across the length of the cell cavity) directly between the first smaller end-mirror and the larger opposing mirror and then (across the length of the cell cavity) directly between the second smaller end-mirror and the larger opposing mirror, in a repeated so-called “V” pattern, before exiting. White cells are generally considered more complex than Herriott cells but find advantages with incoherent sources such as thermal emitters.
Examples of the aforementioned optical multipass cells and multipass systems that incorporate such cells include U.S. patent application Ser. No. 11/671,364 to inventor Walter M. Doyle, which provides a mirror alignment adjustment and a mirrored viewing window based on a White cell. U.S. Pat. No. 5,440,143 to inventors Carangelo, et al. further provides a gas cell, also based on a White cell, in which at least one of the end-mirrors is formed with an added cylindrical component. U.S. patent application Ser. No. 13/885,178 to inventors Sven Krause et al. provides a gas spectrometer having a cylindrical multipass cell with two opposing end-mirrors in a tubular chamber (i.e., cell cavity) that tapers from a gas inlet end toward a gas outlet end. U.S. patent application Ser. No. 10/081,655 to inventors James T. Daly et al. provides an absorption spectroscopy apparatus incorporating a multipass cell that has a curved reflective inner side wall surface; here, a beam of energy is reflected off the reflective surface axially in substantially the same plane inside the cell. U.S. Pat. No. 5,291,265 to Kebabian employs astigmatic mirrors, and provides for mirror manufacturing errors to be resolved by adjusting mirror separation and twist angle. U.S. Pat. No. 7,800,751 to inventors Silver, et al. provide a two end-mirror optical cell and method employing cylindrically curved mirrors; the cell has an inlet hole on the first end-mirror and an outlet hole at approximately the center of an opposing second end-mirror, forming a lissajous spot pattern on the end-mirrors. Finally, U.S. Pat. No. 8,531,659 to inventors Stephen So, et al. employs spherical aberration to advantage, and utilizes an “iterative artificial intelligence-based optimization” to create spot patterns for a particular, selected multipass cell configuration.
Unfortunately, the above and other optical multipass systems are found by the present inventors to be unduly limited by the particular multipass cell type, and have heretofore proven incapable of achieving a more compact size, different/variable geometry, lesser complexity and/or useful manipulation of subject samples, sources, detection, computational systems and/or other characteristics. Such deficits not only result in limited multipass system arrangement, size and/or other constraints, but also result in greater cost, for example, where control of temperature, pressure and/or other environmental aspects are exacerbated by greater size, or where portability, operational feasibility and/or other factors are negatively impacted. Requisite use of custom mirrors, grinding and/or other optics can also reduce flexibility, increase cost, and so on, among yet further disadvantages.
Accordingly, there is a need for optical multipass cell, spectrometer and/or other optical multipass systems and methods that enable a long optical path in a very small multipass cell size or variable geometry/configuration. There is also a need for optical multipass systems and methods that enable production of high density optical patterns while avoiding optical interference, that are capable of lower-cost production/operation, and/or that enable combinations of the above and/or other disadvantages of the above and/or other conventional\emerging systems to be avoided and/or further advantages to be achieved.